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Severe Combined Immunodeficiency (SCID)

Updated on May 28, 2019
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Introduction

Severe Combined Immunodeficiency Disease (SCID) represents numerous inherited genetic diseases. The basic clinical symptoms are quite similar in many forms of SCID where the affected individuals exhibiting defective T- and B-cell function. SCID can be subdivided according to the presence or absence of T-cells, B-cells and NK cells. SCID is more common in male individuals. The most common form of SCID is X-linked SCID, which accounts for almost half the cases of SCID. The affected persons have decreased number of B-cells and T-cells in the the body. Again, sometimes the numbers may be normal but the cells may be inactive, meaning that they can function properly. SCID can lead to many other severe diseases like cancer due to the inactivity of immune system.

Pic: Picture showing the difference between a normal human and a diseased human
Pic: Picture showing the difference between a normal human and a diseased human | Source

Clinical presentation of SCID

The signs and symptoms of SCID can be detected usually in a few months of birth of the affected individual. The symptoms include:

1. Viral infections: Cytomegalovirus (CMV), adenovirus, herpes simplex virus, rota virus etc. infections are commonly found in the SCID patients.

2. Crytosporidial infections

3. Candida albicans infections

4. Bacterial infections by S. aureus, P. jiroveci etc.

5. Aspergillus infections

6. Protozoan infections

7. Abnormality in Cell mediated immunity

8. Antibody deficiency

9. Chronic diarrhea

10. Skin rashes

11. Sinopulmonary infections

12. Sepsis

13. Oral and genital ulcers

14. Behavior problems and a decreased IQ

15. LIver infection

16. Meningitis

17. Poor wound healing

18. Frequent episodes of diarrhea, pneumonia, otitis and sepsis.

Since SCID is associated with defection in immune system, so any type of infections can occur because the immune system of that particular individual is weak. Viral, bacterial, fungal infections are common and many other diseases can also occur frequently.

Causes of SCID

1. X-linked SCID is caused by abnormalities in the gamma-c family of cytokines.

2. SCID associated with T-cell defects is caused by the abnormality in the T-cell receptor or CD3 deficiency.

3. SCID associated with enzyme deficiencies is caused by any defect in the adenosine deaminase gene or nucleoside phosphorylase gene.

4. SCID associated with defective HLAb expression is caused by any defect in the gene that express HLA class-II molecules.

In all the cases, the final result is that, the B-cell, T-cell and/or the NK cells will be defected in the diseases individual, resulting in defective immune system.

Genetic Basis of SCID

Pic: Picture showing various genes responsible for SCID.
Pic: Picture showing various genes responsible for SCID. | Source

Genetic Basis of SCID (cont.)

Various genes are responsible for SCID. IL7R gene, IL7R-alpha chain gene, CD45 gene, IL2RG gene, RAG1 & RAG2 genes, ARTEMIS gene, ADA gene etc. are genes that are responsible for SCID. Any defect in any one of these can cause SCID.

Again, defective JAK-STAT pathway also causes SCID.

Mechanisms of How SCID Occurs

There four mechanisms by which SCID occurs.

1. Premature cell death caused by the accumulation of purine metabolites. This phenomena occurs in adenosine deaminase (ADA) deficiency and due to this reason all the lymphocytes are equally affected. ADA deficiency accounts for about 15% of SCID cases.

2. Defective cytokine signaling can lead to SCID. These signals maintains the survival of T-cells and NK cells. Various cytokines, for example: IL-2, IL-4, IL-7 etc. are responsible for this phenomena. This type of SCID is the most frequent, with around 55% of SCID cases.

3. SCID can be caused by defections in the V(D)J gene rearrangement of T-cell receptors and B-cell receptors.

4. Defective pre-TCR and TCR signaling can also lead to SCID.

Pic: Proportions of SCID genotypes diagnosed with or without newborn screening.
Pic: Proportions of SCID genotypes diagnosed with or without newborn screening. | Source

Diagnosis of SCID

The suspected patient requires evaluation of both humoral and cellular immunity. Flow cytometric immunophenotyping of lymphocyte subsets in peripheral blood is important in patients suspected of having SCID. Again, evaluation of the number of active/naive T-cell in the peripheral blood can be done as a diagnostic procedure, since all types of SCID are characterized by a severe deficiency in naive T-cells. Moreover, the evaluation of humoral immunity may include measurement of Ig levels.

Treatment of SCID

1. Enzyme replacement therapy: In enzyme replacement therapy, ADA enzyme coupled to polyethylene glycol, is injected intramuscularly. These injections should be given once a week for the entire lifetime of the affected individual. This therapy gives the patient with ADA deficiency a better life to lead.

2. Stem cell transplantation: Stem cells are the cells that are collected from the blastula and these cells have the capability of converting to any cell type of the body. In stem cell transplantation, the stem cells are collected from the blastula, genetically engineered in vitro and transplanted into the patients so that the cells may convert into the B-cells, T-cell and NK cells.

3. Gene therapy: In gene therapy, the cells or bone marrow cells are collected from the patient, genetically engineered the cell in vitro so that the cells may express the desired gene that encodes immunologically important molecules, then the cells are transferred back to the patient. This strategy is effective if the pateint lacks any specific gene that is involved with the immunology.

Pic: Gene therapy to treat SCID
Pic: Gene therapy to treat SCID | Source

© 2019 Bishajit Sarkar

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